CN1288506C - Method for manufacturing photoetching equipment and device - Google Patents

Abstract

The invention relates to a displacement measurement system for measuring the position of an optical element in a projection system of a lithographic projection device. The displacement measurement system utilizes the principle of interferometry, including the use of a first diffraction grating mounted on an optical element and a second diffraction grating mounted on a reference frame.

Description

Lithographic apparatus and method of making

technical field

The invention relates to a lithographic projection device, comprising:

Radiation systems for providing projected radiation beams;

a support structure for supporting a patterning device for patterning the projected beam according to a desired pattern;

a substrate stage for holding a substrate; and

a projection system for projecting a patterned beam onto a target portion of a substrate comprising at least one optical element; and

A displacement measurement system for measuring the position of the at least one optical element.

Background technique

The term "patterning device" as used herein should be broadly understood to refer to a device capable of imparting an incident beam of radiation with a patterned cross-section corresponding to the pattern to be produced in a target portion of a substrate; the term "light valve" Also used in this article. Generally, the pattern corresponds to generating in the target part a pattern specific functional layer within a device, such as an integrated circuit or other device (see below). Examples of such patterning devices include:

mask. The concept of masks is well known in lithography and includes, for example, double masks, alternating phase-shift masks, attenuated phase-shift masks, and various hybrid masks. The placement of such masks in the radiation beam causes selective transmission (in the case of transmissive masks) or reflection (in the case of reflective masks) of the radiation impinging on the mask, depending on the pattern on the mask . In the case of a mask, the support structure is typically a mask table, which ensures that the mask can be fixed at the desired position in the incident radiation beam and that the mask can be moved relative to the beam as desired.

Programmable mirror column. An example of such a device is an addressable matrix surface with a viscoelastic control layer and a reflective surface. The basic principle of such devices is that, for example, addressed areas of a reflective surface reflect incident light as diffracted light, while unaddressed areas reflect incident light as undiffracted light. Using suitable filters, said non-diffracted light can be filtered out of the reflected beam, leaving only the diffracted light; thus, the beam is patterned according to the addressing pattern of the addressable matrix surfaces. An alternative example of a programmable mirror array employs a matrix arrangement of micromirrors, each of which can be individually tilted along an axis by applying an appropriate local electric field or using piezoelectric actuation means. Again, these mirrors are an addressable matrix such that the addressed mirrors will reflect the incident radiation beam in a different direction than the non-addressed mirrors; thus the reflected beam is formed according to the addressing pattern of the columns of addressable mirrors pattern. The required addressing matrix can be accomplished using suitable electronics. In both cases above, the patterning device may comprise one or more programmable mirror arrays. Further information on the mirror trains mentioned herein can be obtained, for example, from US Patent Nos. 5,296,891 and 5,523,1932 and PCT Patent Applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied, for example, as a frame or table that is fixed or movable as required.

Programmable LCD array. An example of such a construction is given in US Pat. No. 5,229,872, incorporated herein by reference. As above, the support structure may be embodied as a frame or table that is fixed or movable as required.

For purposes of simplicity, certain other places in the text of this application refer specifically to examples involving masks and mask tables; however, the general principles discussed in these examples should be understood in the broader context of patterning devices as described above. within range.

Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this case, the patterning device generates a circuit pattern equivalent to a single layer of an IC, which can be imaged on a target portion of a substrate (silicon wafer) already coated with a layer of radiation-sensitive material (resist) (e.g. , including one or more moulds). Typically, a single wafer will contain an entire network of adjacent target portions that are successively irradiated by the projection system one at a time. In current setups, two different types of machines can be distinguished by using a mask on a mask table to form a pattern. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern on that target portion at once; such apparatuses are commonly referred to as wafer steppers. In an alternative arrangement—often referred to as a continuous scanning arrangement—the mask pattern is progressively scanned in a given reference direction (“scanning” direction) under the projected beam, while simultaneously The substrate table is scanned synchronously parallel to this given reference direction to illuminate each target portion; since, in general, projection systems have a magnification factor M (typically < 1), the velocity at which the substrate table is scanned is the mask table M times the speed of the scanned place. Further information on the lithographic apparatus described herein can be gleaned from, for example, US6046792, incorporated herein by reference.

In a fabrication method using a lithographic projection device, a pattern (eg in a mask) is imaged on a substrate at least partially covered with a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate can be subjected to various processes such as priming, resist coating, and soft bake. After exposure, the substrate can be subjected to other processes such as post-exposure post-baking (PEB), development, hard-baking, and measurement/inspection of imaged features. This set of processes is used as the basis for patterning a single layer of a device, such as a single layer of an IC. This patterned layer can then be subjected to various treatments like etching, ion implantation (doping), metallization, oxidation, chemical mechanical polishing, etc., all of which are used to complete a single layer. If several layers are required, the whole process or a variation thereof has to be repeated for each new layer. Ultimately, a set of devices is located on a substrate (wafer). Then, the devices are separated from each other by a manipulation technique such as cutting or slicing, whereby individual devices can be mounted on carriers connected by pins or the like. Information about this can be obtained, for example, from the book "Microchip Fabrication: A Practical Guide to Semiconductor Processing", 3rd Edition, Peter van ant, McGraw Hill Press, 1997, ISBN 0-07-067250-4, which is hereby incorporated by reference. Further information on processing.

For purposes of simplicity, projection systems are referred to hereinafter as "lenses"; however, this term should be broadly understood to encompass various types of projection systems including, for example, refractive optics, reflective optics and catadioptric systems. A radiation system includes components, which may also collectively or individually be referred to as "lenses", operate according to any type of design for directing, shaping or controlling a transmitted radiation beam. Furthermore, the lithographic apparatus may be of the type having two or more substrate stages (and/or two or more mask stages). In such "multi-stage" equipment, additional stages may be used in parallel, or preparatory processes may be performed on one or more stages while one or more stages are being used for exposure. For example, dual-stage lithographic apparatus are described in US5969441 and WO98/40791, incorporated herein by reference.

For microlithography for integrated circuits and liquid crystal display panels and other types of devices, one of the most complex requirements is the positioning of the optical elements in the projection system PL. For example, lenses used in conventional lithographic projection setups need to be set to sub-lOnm precision in six degrees of freedom (DOF). EUV lithography setups must use mirror columns in the projection system because no suitable material for forming EUV refractive optics is known and must be kept under vacuum to avoid contamination and attenuation of the beam. At the wavelengths used by EUV systems, positioning accuracies below 0.1 nm are required.

Currently, lenses and mirror columns are positioned on the micron scale across the entire working range by using a coarse positioning actuator, in series with a fine positioning actuator. The latter is used to correct the residual error of the coarse positioning module to the last few nanometers or its mantissa as appropriate, but it can only adjust a very limited range of movement. Typically such actuators include piezoelectric actuators or voice coil type electromagnetic actuators. Although positioning in a fine module is usually done within six degrees of freedom, extensive movements are rarely required for more than two degrees of freedom, greatly simplifying the design of the coarse module.

Using known position detectors like interferometers, the required micrometer accuracy for coarse positioning is easily achieved. These detectors can be single-axis devices, each measuring one degree of freedom. However, these devices are expensive, bulky, not capable of repeatable measurements, and can only measure displacement rather than changes in absolute position.

On the other hand, the position measurement of the optical element at the precise positioning actuator needs to be accurate to 10 nm in all six degrees of freedom. Under current needs, capacitive sensors are used.

With the need for finer resolution, the wavelength of lithographic projection radiation has been reduced to the EUV range of approximately 5 to 20 nm wavelength. Therefore, the required positional accuracy becomes more precise. It has been found that the accuracy required for position measurement cannot be obtained with capacitive sensors because the rotation and displacement of the capacitive sensor cannot be distinguished. Additionally, capacitive sensors are not thermally stable over their entire operating range.

Contents of the invention

Therefore, in EUV lithography projection setups, there is a need for a displacement measurement system with higher resolution than previously used capacitive sensors that is not only compact but also capable of measuring the position of optical elements in all six degrees of freedom. These sensors also need to be insensitive to temperature fluctuations.

It is an object of the present invention to provide an improved displacement measurement system for use in lithographic projection apparatus, in particular a system which solves or improves upon the problems encountered with existing systems.

According to the invention, the above and other objects are achieved in a lithographic apparatus as described in the opening paragraph, characterized in that said displacement measurement system comprises a first diffraction grating mounted on at least one optical element, and mounted on a reference An associated second diffraction grating on the frame, wherein one of said first and second diffraction gratings is arranged to receive diffracted light from the other diffraction grating.

In this way, the position of the optical element on which the first diffraction grating is mounted can be reliably measured within one degree of freedom using the principle of interferometry capable of obtaining an accuracy equal to 0.1 nm. When the first diffraction grating is moved relative to the second diffraction grating, a phase difference in light waves is produced by the grating arranged to receive light diffracted from the other diffraction grating. These resulting phase differences are proportional to the displacement of the diffraction grating relative to the other, whereby the measured results can be used to accurately measure the position of the optical element under the principle of interferometry using single-field scanning.

The displacement measurement system of the present invention is based on the principles described by SPIES, A. in the book "Linear and Angle Encoders for the High Resolution Range", Advances in Precision Engineering and Nanotechnology, Braunschweig, 1997, cited herein Reference. Similar encoders are commercially available, eg the interferometric linear encoder LIP382 of Dr. Johannes Heidenhain Gmbh, Traunreut, Germany.

In addition to the good accuracy of the displacement measurement system of the present invention, the system can be made compact and vacuum and temperature stable through careful selection of materials for the various components.

In a preferred embodiment, each diffraction grating has an associated grating pattern with fiducial marks for determining the fiducial position of the movable object. In this way, the absolute position of the movable target can be measured.

In a preferred embodiment, the displacement measurement system further comprises a light source for generating source light, said displacement measurement system being arranged such that said light is diffracted by one of said first and second diffraction gratings, and thereby generating a first diffracted light signal which is diffracted by the other of said first and second diffraction gratings and thereby generating a second diffracted light signal which is diffracted by said first and the one of the second diffraction gratings to generate a third diffracted optical signal. Preferably, one of said first and second diffraction gratings is a transparent diffraction grating and said other of said first and second diffraction gratings is a reflective diffraction grating. In this way, the displacement measuring means can be kept small, and both the light source and any light sensor can be positioned next to each other by one of said first and second diffraction gratings.

Preferably, the displacement measurement system comprises at least two first diffraction gratings and at least two second diffraction gratings, the first and second diffraction gratings connected in pairs are installed substantially orthogonally. In this way, the position of the optical element can be measured in two degrees of freedom. Obviously, by providing a pair of first and second diffraction gratings for each degree of freedom, it is possible to measure the position of the optical element in all six degrees of freedom.

According to another aspect of the present invention, a method for manufacturing a device is provided, comprising the steps of:

providing a substrate at least partially covered by a layer of radiation-sensitive material;

using a radiation system to provide the projected radiation beam;

using a patterning device to impart a pattern to the projected beam in its cross-section;

projecting a patterned beam of radiation onto a target portion of the layer of radiation-sensitive material using at least one optical element; and

measuring the position of the at least one optical element;

It is characterized in that: a first diffraction grating installed on the at least one optical element and a second diffraction grating installed on the reference frame are provided; and another diffraction from the second diffraction grating.

Although the use of devices according to the invention in the fabrication of ICs is specifically referred to herein, it should be clearly understood that there are many other possible applications for such devices. For example, it can be used in integrated optical systems, guidance and detection of magnetic domain memories, liquid crystal display panels, thin film magnetic heads, etc. Those of ordinary skill will appreciate that within the scope of such alternative applications, any use of the terms "reticle," "wafer," or "mold" herein may be considered to be replaced by the more general terms "mask," "substrate," or "mold." Slice" and "Target Section".

As used herein, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365, 248, 193, 157, or 126 nm) and extreme ultraviolet (EUV) radiation, such as With wavelengths in the range of 5-20nm, especially around 13nm), and particle beams, like ion beams or electron beams.

Description of drawings

Embodiments of the invention are now described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic projection apparatus according to one embodiment of the present invention;

Figure 2 illustrates the juxtaposition of optical elements, reference frame and displacement measurement system of the present invention;

Figure 3 depicts a displacement measurement system of the present invention.

Corresponding reference characters indicate corresponding parts throughout the drawings.

Detailed ways

Figure 1 schematically depicts a lithographic projection apparatus according to a particular embodiment of the invention. The unit includes:

a radiation system Ex, IL for providing a projected radiation (e.g. EUV radiation) beam PB, which in this particular case also includes a radiation source LA;

A first target table (mask table) MT is provided with a mask holder for holding the mask MA (eg reticle) and is connected to a first positioning device for precise positioning of the mask relative to the part PL:

The second target stage (substrate stage) WT is provided with a substrate holder for fixing a substrate W (for example, a resist-coated silicon wafer), and is connected to a second positioning device for precisely positioning the substrate with the relevant part PL. on PW;

A projection system PL (eg mirror array) is used to image the radiation portion of the mask MA onto a target portion C of the substrate W (eg comprising one or more molds).

As described herein, the device is reflective (ie, with a reflective mask). Usually, however, it can also already be, for example, transmissive (with a transmissive mask). Alternatively, the device may also employ another type of patterning device, such as a programmable mirror array of the type described above.

A radiation source LA (eg a laser generating source or a plasma emitting source) generates a radiation beam. The radiation beam is introduced into the illumination system (illuminator) IL either directly or after passing through a lateral adjustment device such as a beam expander Ex. The illuminator IL may comprise adjustment means AM for setting the outer and/or inner radiation range (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the radiation beam. In addition, it typically also includes various other components like an integrator IN and a condenser CO. Thus, the radiation beam PB impinging on the mask MA has ideal uniformity and intensity distribution in the cross section.

With regard to FIG. 1, it should be noted that the radiation source LA may be located within the housing of the lithographic projection apparatus (as is often the case, for example, in the case of a mercury lamp), but that the radiation source LA may also be remote from the lithographic projection apparatus, where The resulting radiation beam is introduced into the device (for example with the aid of suitable guiding mirrors); this latter is often the case when the radiation source LA is an excimer laser. The present invention and claims encompass both cases.

The radiation beam PB then intersects the mask MA fixed on the mask table MT. Radiation beam PB is focused on target portion C of substrate W after passing through projection system PL after being selectively reflected by mask MA. With the aid of the second positioning means (and the interferometric means IF), the substrate table WT can then be moved precisely, for example with different target portions C in the optical path of the radiation beam PB. Similarly, the first positioning means can precisely position the mask MA according to the optical path of the radiation beam PB; for example, after the mask MA is mechanically withdrawn from the mask library, or during scanning. Generally, the movement of the target tables MT, WT can be realized with the assistance of a large stroke module (coarse positioning) and a small stroke module (fine positioning), which is not clearly depicted in FIG. 1 . However, in the case of a wafer stepper (as opposed to a step-and-scan device), the mask table MT may only be connected to a short impact actuator, or be fixed.

The device described can be used in two different modes:

1. In step mode, the mask table remains essentially stationary and the entire mask image is projected on the target portion C in one go (ie, one "flash"). The substrate table WT is then moved in the x and/y directions so that other target portions C are irradiated by the radiation beam PB.

2. In scan mode, essentially the same situation, except that given target portion C is not a "flash" exposure. Instead, the mask table MT is moved with a velocity v in a given direction (the so-called "scanning direction", e.g. the y-direction) such that the projection beam PB is caused to scan the entire mask image; Move in the same or opposite direction at a velocity of V=Mv, where M is the magnification of the lens PL (typically, M=1/4 or 1/5). In this way, a relatively large target portion C can be exposed without compromising resolution.

In a lithographic apparatus using radiation at a wavelength of 157 nm, the projection system PL has one or more optical elements (lenses) that need to be precisely positioned relative to the reference frame 30 . With the advent of EUV technology, mirror columns have to be used exclusively instead of lenses to focus the beam PB in the projection system PL. Therefore, not only does the EUV optic within the projection system PL require a higher positioning accuracy because of the use of shorter radiation wavelengths, but it also requires a higher two positioning accuracy because it is used in reflective rather than refractive mode. One of the factors of precision. Due to the increased resolution, a correspondingly higher accuracy is required. Thus, although not exclusively, this is essential for the EUV type lithography apparatus to which the present invention is concerned.

Optical elements in projection systems PL often need to be positioned with six degrees of freedom. In the case of the optical elements in the EUV device projection system PL, the positioning accuracy is on the order of 0.1 nm. A typical EUV setup includes six mirrors in projection system PL, each positioned with six degrees of freedom relative to a frame 30, which serves as a reference frame.

Figure 2 shows the coupling of the optical element 20 relative to the reference frame 30 by means of an actuator 40 (such an arrangement is disclosed eg in European Patent Application No. 01310781.8, incorporated herein by reference). Meanwhile, the displacement measurement system 50 is also illustrated in the figure. A part of the displacement measurement system 50 is mounted on the reference frame 30, and another part of the displacement measurement system 50 is mounted on the optical element 20, which is a projection target box 30 in the present embodiment. There is no physical contact between the two parts of the displacement measurement system 50 .

The displacement measurement system 50 will now be described in more detail with reference to FIGS. 2 and 3 . The displacement measurement system includes a first diffraction grating 51 mounted on the optical element 20 . This can be achieved by etching a diffraction pattern on the surface of the optical element 20 or by bonding the diffraction grating molecules to the surface of the mirror. In the latter case, the diffraction grating can be made of a low thermal expansion material like Zerodur (RTM) or ULE (RTM), and the diffraction pattern can be chromium lines. A grating period of 512 nm is suitable, while the diffraction grating is a reflective diffraction grating.

The second major component of the displacement measurement system 50 is a second diffraction grating 52 fixed to the projection target box 30 . In the illustrated embodiment, the second diffraction grating is a transmissive diffraction grating with a grating period of 512 nm, ie the same as the first diffraction grating but the period could be greater. Preferably, the second diffraction grating is also made of a low thermal expansion material like Zerodur (RTM) or ULE (RTM).

These diffraction gratings can also be fabricated in the same way as reticles in lithographic projection setups. Although the position of the second diffraction grating 52 is fixed relative to the reference frame (ie, the transmission target box 30 ), the length of the second diffraction grating 52 may be shorter than the length of the first diffraction grating 51 . In fact, the first diffraction grating 51 is selected to match the required amount of movement of the optical element 20 . In EUV systems it is about 1mm.

Although the displacement measurement system 50 has been described with a first diffraction grating 51 mounted on the optical element 20 and an associated second diffraction grating 52 mounted on the projection target box 30, this is not essential and the first The diffraction grating 51 may also be mounted on the projection target box 30 , while the second diffraction grating 52 is mounted on the optical element 20 . However, the illustrated embodiment is preferred because the displacement measurement system 50 described here has the additional weight of its optics carried by the projection target box and not transferred through the actuator 40 . Therefore, the inertia of the optical element 20 is not significantly increased by using the displacement measurement system 50 .

Other components fixedly connected to the projection target box 30 and the second diffraction grating 52 are a light source 53 and a sensing system 54 comprising three light sensors (diodes) 54a, 54b and 54c. The light source 53 can be remote from the displacement measurement system to avoid localized heating. Light can be delivered to the displacement measurement system by means of optical fibers. A lens 55 is arranged between the second diffraction grating 52 and the light source 53 and the induction system 54, and is fixed at an appropriate position relative to the projection target box 30 for focusing the light from the light source 53 to the second diffraction grating 52 and The diffracted light from the second diffraction grating 52 is focused to sensors 54a, 54b and 54c.

The functional principle of the inventive displacement measuring device, which can also be called an interferometric linear encoder, is in SPIES, A. "Linear and Angular Encoders for the High Resolution Range", Advances in Precision Engineering and Nanotechnology, Braunschweig, 1997 described in detail.

A light source 53 provides a collimated beam which is perpendicular to the measurement direction and directed towards a second diffraction grating 52 where it is diffracted into third order. The zeroth order is hidden, and only the +/-1st order beams pass to the first diffraction grating 51 as the first diffraction signal. At the first diffraction grating 51, the first diffracted optical signal is diffracted in a Littrow arrangement and reflected to form a second diffracted optical signal. The second diffracted light signal interferes on the second diffraction grating 52 and is diffracted as it passes through the second diffraction grating 52 to form a third diffracted light signal. The third diffracted light signal is focused by a lens 55 to the three light detectors 54a, 54b, 54c of the sensing system.

The signal received by each photodetector 54a, 54b, 54c is 120° phase shifted from the light signal received by the other two photodetectors 54a, 54b, 54c. If there is no relative movement of the first diffraction grating 51 with respect to the second diffraction grating 52, the outputs of these sensors are constant.

The electrical signals from the inductive system are converted into quadrature signals by electronic circuits. If the first diffraction grating 51 is offset relative to the second diffraction grating 52, the outputs of the sensors 54a, 54b, 54c oscillate. When one diffraction grating is moved relative to the other, the sine and cosine signals output by the circuit have four times the period of the diffraction grating (since it is a two-way pass in a transparent grating setup, the electrical signal has a period of is twice the grating period). Thus, the insertion of electrons, for example by a factor of 214, can easily lead to measurement steps below 0.1 nm.

The first diffraction grating 51 may have one or more fiducial marks adjacent to the first diffraction grating 51 and corresponding marks adjacent to the second diffraction grating 52, so that the absolute position of the optical element 20 can be determined by another photodetector (not shown). ) detection. These fiducial marks can be used to generate separate fiducial signals.

As will be appreciated, by using more pairs of associated first 51 and second 52 diffraction gratings arranged orthogonally to other pairs, position measurement of the optical element 20 in more than one degree of freedom is possible. Two orthogonal pairs of diffraction gratings may be provided on a single housing.

Also, it should be understood that other optical geometries can be used in the displacement measurement system. For example, the collimated light beam from the light source 53 can be directed to the second diffraction grating 52 at an oblique angle, and a prism located between the first diffraction grating 51 and the second diffraction grating 52 can be used to combine the first and second diffraction gratings. The optical signals are reflected to the first and second diffraction gratings, respectively. Also, the second diffraction grating 52 can also be a reflective diffraction grating, and many other different geometries of the same principle as above are also available.

While specific embodiments of the invention have been described above, it is to be understood that the invention may be practiced otherwise than as described. This description is not intended to limit the invention.

Claims (10)

1. A lithography projection device, comprising: a radiation system for providing a projected radiation beam; a support structure for supporting a patterning device for patterning the projected beam according to a desired pattern; A substrate table, used for fixing the substrate; a projection system comprising at least one optical element for projecting a patterned light beam onto a target portion of the substrate; and a displacement measurement system for measuring the position of said at least one optical element; It is characterized in that the displacement measurement system includes a first diffraction grating installed on the at least one optical element and a second diffraction grating connected on the reference frame, wherein the first and second diffraction gratings One is arranged to receive diffracted light from the other diffraction grating. 2. Apparatus according to claim 1, wherein each diffraction grating has one or more associated fiducial marks for determining the fiducial position of said optical element. 3. The device according to claim 1, wherein said displacement measuring system is arranged such that light from a light source is diffracted by one of said first and second diffraction gratings, and thereby produces a first diffracted light signal, the first The diffracted light signal is diffracted by the other of the first and second diffraction gratings, thereby generating a second diffracted light signal that is diffracted by the one of the first and second diffraction gratings Diffraction, thereby generating a third diffracted optical signal. 4. The apparatus according to claim 3, wherein said one of said first and second diffraction gratings is a transmission diffraction grating and said other of said first and second diffraction gratings is a reflection diffraction grating. 5. The device according to claim 4, wherein said reflective diffraction grating is mounted on said optical element. 6. Apparatus according to claim 3, 4 or 5, wherein said displacement measurement system further comprises a sensing system for receiving said third diffracted light signal. 7. The device according to claim 6, wherein said sensing system comprises three light sensors, each of which is adapted to receive light signals with a phase shift of 120° from the signals received by the photosensitivity of said three light sensors . 8. The apparatus according to claim 6, wherein said displacement measurement system includes a processor for converting signals from said sensing system into position information of said optical element with respect to a reference frame by interpolation. 9. The device according to claim 1, 2, 3, 4 or 5, wherein said displacement measurement system comprises at least two first diffraction gratings and at least two second diffraction gratings, the first and second diffraction gratings connected in pairs The gratings are mounted orthogonally. 10. A method of manufacturing a device, comprising the steps of: providing a substrate at least partially covered by a layer of radiation-sensitive material; using a radiation system to provide the projected radiation beam; using a patterning device to impart a pattern to the projected beam in its cross-section; projecting a patterned beam of radiation onto a target portion of the layer of radiation-sensitive material using at least one optical element; and measuring the position of the at least one optical element; providing a measurement system for measuring the position of the at least one optical element It is characterized by: The measurement system includes a first diffraction grating mounted on the at least one optical element and a second diffraction grating mounted on a reference frame; light diffracted by one of the first and second diffraction gratings is captured by the first One and another diffraction of the second diffraction grating.

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